四大组件之Service

A 

Service

 is
an application component that can perform long-running operations in the background and does not provide a user interface. Another application component can start a service and it will continue to run in the background even if the user switches to another
application. Additionally, a component can bind to a service to interact with it and even perform interprocess communication (IPC). For example, a service might handle network transactions, play music, perform file I/O, or interact with a content provider,
all from the background.

服务是可以在后台执行长时间运行操作，并且不提供用户界面的应用组件。另一个应用程序组件可以启动服务，它将继续存在即使用户切换到另一个应用程序在后台运行。此外，一个组件可以绑定到一个服务来与它交流，甚至进行进程间通信（IPC）。例如，服务可能处理网络交易，播放音乐，执行文件I / O，或与Content Provider进行交互，所有都是从后台运行

A service can essentially take two forms:

服务基本上可以采取两种形式：

StartedA service is "started" when an application component (such as an activity) starts it by calling

startService()

.
Once started, a service can run in the background indefinitely, even if the component that started it is destroyed. Usually, a started service performs a single operation and does not return a result to the caller. For example, it might download or upload
a file over the network. When the operation is done, the service should stop itself.
Started

       服务的“启动”当应用程序组件（如Activity）通过调用startService启动它,一旦开始，一个服务可以在后台运行下去，即使启动它的组件被破坏。一般，服务执行单次操作，其结果不返回给调用者。例如，它可以下载或上传文件在网络上。当操作完成时，服务应停止本身。